In an image forming apparatus of an electrophotographic process, for forming an electrostatic latent image on a charged electrophotographic photoreceptor, there is known a method of scanning the electrophotographic photoreceptor with plural light beams (hereinafter called “multi-beam method”). See, for example, patent document 1.
Patent Document 1: JP 2002-303997 A
The image forming apparatus of such multi-beam method is considered advantageous for elevating the speed of an image forming process, but is not necessarily satisfactory in the image quality, particularly in the case of employing a surface emitting laser array capable of increasing the number of lasers. More specifically, an electrostatic latent image formed on the electrophotographic photoreceptor includes areas with different numbers of scanning (i.e., number of irradiations)) by the light beam until the end of the exposure, and such difference in the number of irradiations between such areas may result in observation of a streak-shaped density unevenness.
FIG. 11 is a chart showing a distribution of an exposure energy along a moving direction (sub-scanning direction) of the electrophotographic photoreceptor in the case where 30 laser beams of a spot diameter of 50 μm are made to scan an electrophotographic photoreceptor to perform scanning of simultaneous 30 scan lines (with scanning line density of 2600 dpi (the term “dpi” means dot per inch)) per one main scan and the electrophotographic photoreceptor is moved to shift the scanning lines by a distance equivalent to 30 scanning lines for every main scan.
As shown in the drawing, the exposure energy distribution in each main scanning becomes approximately trapezoidal. Amoung the exposure energy distribution given to the electrophotographic photoreceptor in each main scan, a flat part corresponding to a top portion of the trapezoid is an area where the total exposure energy is given by a single exposure (single exposure area), while a sloped part of the trapezoid corresponds to an area where the total exposure energy is given by two exposures (multiple exposure area).
According to an investigation of the present inventors, even in the case where the total exposure energy in the multiple exposure area is equal to that of the single exposure area, an actually obtained image shows a higher image density in the multiple exposure area than in the single exposure area, whereby a streak-shaped density unevenness is generated.